Dual band switched-parasitic wire antennas for communications and direction finding

Wire dipole and monopole switched-parasitic antennas have been designed to operate at 900 and 1900 MHz simultaneously. The array consists of a centre driven element circled by four parasitic elements symmetrically arranged. DC levels applied to p.i.n. diodes on each parasitic element allow full 360 degree coverage in the horizontal plane. The design of the antenna was undertaken using the genetic algorithm technique with the NEC2 solver. The design was optimised for element spacing and lumped impedance size and position. The cost function included the input impedance and directional characteristics at both frequencies. The final design has a front to back ratio of approximately 15 dB, S/sub 11/ less than -35 dB and 360 degree coverage of within 1.5 dB of the main lobe gain for both frequencies.